Alice is a rational-minded sleep doctor but has always been haunted by the mysterious death of her younger brother. While making an examination on a family with sleeping problems, Alice is attacked by the father in his sleep. Deemed as the cause of his family's sleep issues, the father is arrested and thrown into prison. However, the problem only gets worse for the family, and Alice must now abandon her scientific teaching, accepting that the family is being terrorised by the Night Hag, a parasitic demon that feeds on its victims while they sleep. To fight against the evil spirit, Alice must go deep inside her nightmare and confront her own childhood demons.